The Active Medium Propagation (AMP) device is a planar traveling waveguide in which amplification of a propagating RF wave takes place. Such devices were only recently developed, and a detailed description can be had by referring to U.S. Pat. No. 3,975,690 and to P. L. Fleming, "The Active Medium Propagation Device," Proc. IEEE, Volume 63, No. 8, pp. 1253-1254, August 1975. This invention relates to the coplanar version of the AMP device. Briefly described, the coplanar AMP device comprises a set of three conductors applied to the surface of an epitaxial layer of lightly doped (.congruent.10.sup.15 cm.sup.-3), n-type GaAs which was grown on a substrate of semi-insulating GaAs. When the conductors are biased to provide a high enough electric field, electrons in the epitaxial layer are given sufficient energy to promote them into a conduction band valley where their mobility is less than when they are in the lower conduction band valley. This results in a negative value for the differential mobility, and an RF wave propagating in the GaAs will gain in amplitude rather than suffer attentuation. Due to the extremely small dimensions of the center conductor, for example, 7.0.mu., it is extremely difficult to connect the center conductor of a coaxial cable to the center conductor of the AMP device. In order to facilitate such connections, the device can be fabricated having a pair of "launch" regions in which the center conductor is flaired outwardly. The dimensions of the electrodes and the inter-electrode gaps in some AMP devices--e.g., those known as AMP III devices--are such that negative differential mobility cannot be achieved in the launch regions, thus giving rise to the terms "launch" region and "active" region, the former term being defined as above, and the latter term being defined as that central portion of the AMP III device within which a propagating RF signal is amplified. In later devices, known as AMP IV devices, the inter-electrode spacing is such that negative differential mobility can be achieved in both the launch and central portions of the device. Thus, the active region in AMP IV would include the launch regions and the central portions of the device.
FIG. 1 is a top view of a coplanar AMP IV device mounted on a coplanar MIC board. The AMP device includes a center conductor 14 and a pair of ground conductors 15 and 16 spaced from either side of the center conductor to form a pair of gaps 18. The portion 20 of the AMP device located between the dotted lines 22 and 24 is the central portion of the device, and the portions 26 and 28 are the launch regions. The MIC board, which may, for example, consist of an alumina substrate having a copper-plated heat sink on the upper surface, is divided into two outer electrodes 30 and 32 and a pair of center electrodes 34 and 36 by the gaps 38, 40, 42 and 44. The ground conductors 15 and 16 of the AMP device are connected to the outer conductors 30 and 32 of the MIC board by quantities of silver epoxy 46 and 48, respectively, and the center conductor 14 of the AMP device is connected at either end to the center conductors 34 and 36 of the MIC board by means of connecting wires 50. Coaxial cables 48 may be connected to the outer extremities of the MIC board conductors by coaxial launchers 51. The dimensions of the electrodes 14, 15 and 16 and the inter-electrode gaps 18 in the launch regions 26 and 28 of the AMP device are not only designed to facilitate connection of the electrodes to the surrounding RF circuitry, but are also designed to provide a smooth impedance transition between the central portion 20 of the coplanar AMP transmission line and the surrounding circuitry.
FIG. 2 is a cross-sectional view along lines 2--2 of FIG. 1 showing the substrate 10, epitaxial layer 12, outer electrodes 15 and 16 and center electrode 14 of the active region of the AMP device. The ground electrodes 15 and 16 are connected to the copper-plated outer electrodes 30 and 32 at the upper surface of the alumina substrate 52 of the MIC board by quantities of silver epoxy 46 and 48.
The RF signal intended to be amplified in the active region of the AMP device is an even mode signal--i.e., the RF signal applied to the center conductor 14 will give rise to electric field components 54 and 56 between the center electrode 14 and the adjacent ground conductors 15 and 16, respectively. The field components are symmetrical about a plane of symmetry 58 extending vertically through the center of the conductor 16. Since the electric field in the epitaxial layer 12 is dependent upon the conditions within the epitaxial layer, it will be apparent to anyone of ordinary skill in the art that the epitaxial layer must be perfectly uniform in the vicinity of the plane of symmetry 58 in order for a pure even mode signal to exist. In reality, however, slight imperfections and non uniformities often exist in the epitaxial layer, giving rise to an "odd" mode signal--i.e., an RF signal propagating in the epitaxial layer having asymmetrical electric field components. The asymmetrical electric field components of the odd mode signal do not cancel each other, and, therefore, the net electric field 60 will create a potential difference between the ground conductors 15 and 16. The existence of both even and odd mode signals in the AMP device results in a "mixed" mode signal--i.e., the combination of both even and odd mode signals--propagating along the AMP device and MIC board. Since the coaxial launchers 51, for connecting the coaxial cables 49 to the MIC board, are designed to provide impedance matching only for even mode signals, they present a higher reflection to the odd mode signal. The successive reflection and amplification of the odd mode signals in the AMP device results in a odd mode oscillation which interferes with and limits the amplification of the desired even mode signal.
Assuming equal gain for both modes, we can write the amplitude criteria for oscillation in the propagation direction as follows:
For even mode, EQU .vertline.K.sub.1 .vertline.e.gtoreq.1.0 (1)
and for odd mode, EQU .vertline.K.sub.2 .vertline.e.gtoreq.1.0 (2)
where K.sub.1 is the reflection coefficient of the coaxial launchers for even mode signals, K.sub.2 is the reflection coefficient of the coaxial launchers for odd mode signals, .alpha. is the gain factor of the AMP device for both modes, and l is the length of the active region of the AMP device. Since .vertline.K.sub.2 .vertline..gtoreq..vertline.K.sub.1 .vertline., odd mode oscillation on the MIC board can occur, limiting the desired even mode signal.
Assuming that an oscillation cavity is formed between the coaxial launchers at either end of an MIC board having a length of one inch, and we can calculate, in a manner well known in the art, the frequency of TEM-type modes which have an E field maximum at the center of the board. The results are shown in the following table.
TABLE 1 ______________________________________ Coaxial Launcher Separation f(GHz) ______________________________________ .lambda./2 1.905 .lambda. 3.81 3.lambda./2 5.71 2.lambda. 7.62 5.lambda./2 9.52 3.lambda. 11.43 7.lambda./2 13.32 4.lambda. 15.24 9.lambda./2 17.13 5.lambda. 19.05 ______________________________________
Thus, the "cavity" will support oscillations at any of the frequencies in the right-hand column. The AMP IV devices mounted on the MIC board have typically oscillated in the range of 13-14 GHz. As can be seen in Table 1, the 7.lambda./2 mode is a good match to the observed frequencies.
Besides causing oscillation, the odd mode signal may also result in rf radiation from the gaps of the coplanar AMP device. No radiation of the even mode signal will occur since the E field components at all points along the plane of symmetry cancel each other; however, due to the imbalance of the E field components of the odd mode signal, a measurable E field will exist at points distant from the center conductor.